Prior art will be described below with reference to the accompanying drawings, in which:
FIG. 1 is a top plan view of a conventional prior-art apparatus for roll forming purlins from a metal strip,
FIG. 2 is a side view of a part of a prior-art roll forming apparatus,
FIG. 3 is a cross-section of a purlin formed in the prior-art roll forming apparatus of FIG. 2,
FIG. 4 shows how C or U purlins are being formed in a prior-art arrangement, and
FIG. 5 shows how Z purlins are being formed in the prior-art arrangement of FIG. 4.
FIG. 1 illustrates a known apparatus for roll forming purlins from a metal strip 1 of thin metal sheet. The metal strip 1 enters the roll former from the left (arrow A), and a purlin 1' formed from the strip 1 leaves the roll former to the right. The forming of the strip 1 in the roll former is illustrated by three cross-sections marked with braces in FIG. 1. In this case, the roll former comprises two sets of rollers, the first set 2 bending the two outermost edges of the purlin 1' to 90.degree. (=the outermost corners inwards by 90.degree.).
After this, the second set of rollers 3 of the roll former bends the innermost sides of the purlin 1' to 90.degree. (=the innermost corners inwards by 90.degree.). The finished U or C profiled purlin 1' will leave the roll former to the right.
FIG. 2 shows the design of sets of freely rotatable or idling rollers in a prior-art roll former, and FIG. 3 shows the cross-section of the purlin 1'. In FIG. 2, the set of rollers of the roll former comprises rollers 5-12 rotatably mounted on a frame 4. The operation of the roll former is based on the fact that, as the horizontal and schematically shown metal strip 1 is fed into the set of rollers 5-12 from the left, it will first reach the roller 5, which makes the first bending of the strip 1. Subsequently, the strip 1 reaches the following roller 6, which in turn bends the strip 1 slightly more. Each following roller 7-11 in the set of rollers will bend the strip 1 slightly more, until the final roller 12 bends the strip 1 in a desired final 90.degree. angle (cf. FIG. 3).
There are a number of previously known roll forming machines for manufacturing U or C purlins. There are also various known roll forming machines for Z purlins. In this case, the bendings in the roll forming line must be feasible not only upwards but also downwards. FIGS. 9A-9E of U.S. Pat. No. 4,716,754 to Youngs, for instance, illustrate a known solution for forming Z purlins. As to the background art, U.S. Pat. No. 4,787,232 to Hayes should also be mentioned, which discloses a roll former for the manufacture of purlins of various profiles.
The degree of difficulty is increased by the fact that U, C and also Z purlins need to be manufactured in the same roll forming line. Commonly available solutions to this problem have been based on changing of the individual rollers or frame members, such as rafts or cassettes, on which the sets of rollers are already mounted. Such changes are laborious and require skilful operators, and special lifting equipment is needed.
FIG. 4 shows a prior-art roll forming arrangement as C or U purlins are being formed. This known arrangement comprises a left-hand and a right-hand roll forming block 13, 17, respectively, drive rollers 14, 18 and sets of rotatable rollers 15, 16, 19. The illustrated prior-art solution can be used to form the metal strip 20 into C or U purlins in the normal state.
FIG. 5 illustrates the prior-art roll forming arrangement of FIG. 4 as Z purlins are being formed. When Z purlins are being formed, the left-hand roll forming block 13 has been lifted vertically upwards (arrow B) relative to the right-hand roll forming block 17 which normally is stationary. The metal strip 21 to be formed will now be differently positioned relative to the drive rollers 14, 18 and to the rotatable rollers 15, 19. In addition, lifting the entire forming block 17 will require separate, complex and heavy-duty lifting equipment. The illustrated prior-art solution will form Z purlins in this state.
The prior-art solution illustrated in FIGS. 4-5 has the drawback of a complex design, involving up to three shafts. On top of that, the lifting of the entire forming block 13 is extremely awkward.
The arrangement shown in FIGS. 4-5 is described in an article by Karl D. Forth entitled "Split-level roll former speeds changeover" issued in February 1996. These known roll formers are marketed by the US company Bradbury Co.
In a further known solution, the pair of rollers turns 180.degree. about its forming centre. The rotating shafts require high accuracy of manufacture and advanced equipment, by means of which the rotation is controlled and blocked (cf. U.S. Pat. No. 4,787,232 to Hayes).